A type of such control apparatuses set forth above is disclosed in U.S. Pat. No. 8,018,185 B2 corresponding to Japanese Patent Application Publication No. 2009-232531. An example of the type of the control apparatuses is designed to determine a norm of an output voltage vector of an inverter in a two-phase rotating coordinate system defined in a rotor of a three-phase motor using a map; the inverter serves as a circuit for applying a variable output voltage to the three-phase motor. The map represents a relationship between a variable of the norm of the output voltage vector, a variable of a request torque for the three-phase motor, and a variable of a rotational speed thereof. For example, the control apparatus determines a waveform of each of drive signals for the inverter according to a modulation factor; the modulation factor is determined based on the determined norm of the output voltage vector of the inverter and an input voltage for the inverter. The phase of the output voltage of the inverter is set as a manipulated variable for feedback control of an actual torque of the three-phase motor.
As a result, each of the drive signals for the inverter is determined based on the phase of the output voltage of the inverter, and a value of the rotational angle of the three-phase motor measured by an angle sensor, such as a resolver; the phase serves as a manipulated variable for feedback control of the actual torque of the three-phase motor.